1. Field of the Invention
The present invention relates to an analytical method of substances and more particularly, to a method of analyzing substances existing in a gas (e.g., air) to find and identify contaminants existing therein, which is preferably used for evaluating the environment or atmosphere in a cleanroom that has been popularly used in fabrication of semiconductor devices.
2. Description of the Prior Art
Usually, the air existing in a cleanroom used for fabrication of semiconductor devices contains various organic substances that apply bad effects to the devices. For example, if some organic substances (i.e., organic contaminants) are adhered onto a single-crystal silicon (Si) wafer, the adhered substances or contaminants may lower the dielectric strength of silicon dioxide (SiO.sub.2) films provided for the electronic elements (e.g., transistors) formed on or over the Si wafer. Alternately, the adhered substances or contaminants may weaken the cleaning effects of cleaning chemicals used in photolithography and etching processes or the like, resulting in insufficient removal of native oxides and/or metallic impurities.
Conventionally, the organic substances existing in the air in the cleanroom, which serve as contaminants for the popular Si devices, have been measured or identified in the following way.
First, a resin-based absorbent is packed into an absorption tube and then, the air existing in the cleanroom is injected into the tube as a sample by using a proper pumping system. Thus, organic substances contained in the injected sample air are absorbed or trapped by the absorbent. Thereafter, the absorption tube is heated to thermally desorb the absorbed substances from the absorbent at a high temperature of approximately 250.degree. C. to 300.degree. C.
Subsequently, the thermally-desorbed organic substances are analyzed by the use of a gas chromatograph and a mass spectrometer. Specifically, the thermally-desorbed substances in the absorption tube are first sent to the gas chromatograph by a carrier gas and physically separated therein. Next, the substances thus physically separated are sent to the mass spectrometer and analyzed qualitatively and quantitatively therein. As a result, the organic substances existing in the sample air are identified.
With the above-described conventional method, a lot of undesired organic substances that apply no bad effect to the semiconductor devices are detected. This is because the resin-based absorbent used in the conventional method is made of an organic material having a property to absorb almost all the existing organic substances. Therefore, the desorbed organic substances from the absorbent are likely to contain the undesired organic substances.
The undesired organic substances serve as a noise source in the analysis for detecting the desired organic substances that apply some bad effect to the semiconductor devices. In other words, the undesired organic substances cause insufficient physical separation of the organic substances to be analyzed in the gas chromatograph, resulting in incorrect qualitative and quantitative analysis in the mass spectrometer.
Consequently, there arises a problem that the desired organic substances are unable to be analyzed with satisfactory correctness.
Moreover, to thermally desorb the absorbed organic substances from the absorbent efficiently, it is preferred that the thermally desorbing temperature is as high as possible. However, because the resin-based absorbent used in the conventional method is usually made of an organic material, the thermally desorbing temperature needs to be set at a comparatively low temperature of approximately 250 to 300.degree. C.
Thus, there arises another problem that the thermally desorbing temperature is unable to be raised as desired.